1. Field of the Invention
The present invention relates to a power factor correction circuit (hereinafter referred to as a PFC circuit), more particularly to an active power factor correction circuit.
2. Description of the Related Art
Generally, there are two types of PFC circuits, active and passive. However, passive PFC circuits have been eliminated from the competition gradually, due to the large layout area of the passive PFC circuit and a power factor below 0.95, with the harmonic waves thereof being large. Therefore, passive PFC circuits have been replaced by active PFC circuits. As shown in FIG. 1a, a conventional active PFC circuit 10 comprises a bridge rectifier that consists of four diodes D1, D2, D3 and D4, and a voltage source Vs, an inductor L1, a diode D5, a capacitor C1 and a switching device S1. By switching the switching device in high frequency appropriately, the active PFC circuit attains a high power factor.
The operation of the conventional active PFC circuit, as shown in FIG. 1a, is described as follows. When the voltage at node A1 is positive, the voltage at node B1 is negative, and the switching device S1 is off, the main current I flows to the capacitor C1 (or load) through the diode D1, the inductor L1, and diode D5, and then flows back to the voltage source Vs through the diode D4, as shown in FIG. 1b. When the switching device S1 is on, the main current I flows from node A1 to node B1 though diode D1, the inductor L1, the switching device S1, and the diode D4, as shown in FIG. 1c. A power factor of more than 0.99 can be achieved by the above method.
Although the power factor can be increased to more than 0.99 by switching the switching device in high frequency, the switching device inevitably generates power dissipation (or loss) when it turns on and turns off, thus degrading the efficiency of the PFC circuit and wasting energy. Also, the temperature raised due to the operation of the switching device will damage the elements in the PFC circuit.
Consequently, a PFC circuit with a snubber circuit is disclosed to reduce the power loss due to the operation of the switching device, as shown in FIG. 2a. The PFC circuit 20 comprises a bridge rectifier coupled to a voltage source Vs, an inductor L2, a diode D26, a capacitor C2, a switching device S2, and a snubber circuit 210 connected in parallel with diode D26 and the switching device S2. As shown in FIG. 2a, the bridge rectifier consists of four diodes D21, D22, D23 and D24. The snubber circuit 210 consists of an inductor Lr2, a diode D28, and a switching device Sa2 connected in series, two diodes D25 and D27, and a capacitor Cr2. The switching device S2 can carry out zero-voltage switching operation to avoid power dissipation, in conjunction with the snubber circuit 210.
The operation of the conventional active PFC circuit 20, as shown in FIG. 2a, is described as follows. When the voltage at node A2 is positive and the voltage at node B2 is negative, and the switching device S2 is off, the main current I2 flows to the capacitor C2 (or load) through the diode D21, the inductor L2 and diode D26, and then flows back to the voltage source Vs though the diode D24, as shown in FIG. 2b. 
Referring to FIG. 2c, when both switching devices S2 and Sa2 are off, the main current I2 flows as described in FIG. 2b. The current Io equals the main current I2, therefore the current Ir is zero. Before the switching device S2 turns on, the switching device Sa2 must turn on first. When the switching device Sa2 turns on, a voltage across the inductor Lr2 equals the voltage on the capacitor C2. Consequently, the current on the inductor Lr2 increases from zero slowly. When the current Ir equals to the main current I2, based on Kirchoff""s Law, the current Io becomes zero. Namely, the diode D26 is off. At this time, the capacitor Cs2 and inductor Lr2 start to resonate. Until the voltage on the capacitor Cs2 decreases to zero, the switching device S2 can then be turned on, so the switching device S2 has no power loss during the switching period.
As shown in FIG. 2d, the main current I2 flows from node A2 to node B2 through the diode D21, the inductor L2, the switching device S2, and diode D24. By the above operation, the switching device S2 dissipates no power during its switching period, due to zero-voltage switching operation, and a high power factor is also obtained.
After the switching device S2 turns on, the energy stroed in the inductor Lr2 charges the capacitor Cr2 through the diode D25 when the switching device Sa2 turns off, as shown in FIG. 2d. When the current Ir decreases to zero, the diode D25 turns off. Consequently, the switching device Sa2 is soft-switched off and the diode D25 is soft-switched on and off.
However, the main current (I or I2) of the PFC circuit (without or with a snubber circuit), must flow through at least three power electronic devices. Namely, as shown in FIG. 1b, the main current I of the PFC circuit 10 flows though diode D1, D5 and D4 when the switching device S1 is off. As shown in FIG. 1c, the main current I of the PFC circuit 10 flows through diode D1, the switching device S1 and diode D4 when the switching device S1 is on. Further, the main current I2 of the PFC circuit 20, as shown in FIG. 2b, flows though diode D21, D26 and D24 when the switching device S2 is off. As shown in FIG. 2c, the main current I2 of the PFC circuit 20 flows through diode D21, the switching device S2 and diode D24 when the switching device S2 is on. The more power electronic elements the main current (I or I2) flows through, the more power dissipation is generated, therefore resulting in poor efficiency in energy transformation.
It is therefore an object of the present invention to provide an active PFC circuit, making the main current only flow through two power electronic elements using two switching devices, thereby reducing power consumption and improving the efficiency of the PFC circuit.
The other object of the present invention is to provide a soft-switched active PFC circuit, wherein the main current not only flows through two electric elements, but also avoids power loss due to switching, thereby improving the efficiency.
The present invention achieves the above-indicated objects by providing an active PFC circuit for improving the efficiency of a voltage source with first and second terminals, comprising the following structure.
An inductor having an terminal coupled to the first terminal of the voltage source.
A first rectifying device having an anode coupled to the other terminal of the inductor, and a cathode.
A second rectifying device having a cathode coupled to the cathode of the first rectifying device, and an anode;
A first switching device having a first terminal coupled to the anode of the first rectifying device, and a second terminal.
A second switching device, having a first terminal coupled to the anode of the second rectifying device and the second terminal of the voltage source, and a second terminal.
A capacitor having two terminals coupled to the cathode of the second rectifying device and the second terminal of the second switching device respectively.
Further, the present invention also provides a soft-switched active PFC circuit for improving the efficiency of a voltage source with first and second terminals, comprising a first module and a second module. The structure and function of the first module are identical to the PFC circuit described above according to the present invention.
The main object of the second module is to make the first and second switching devices S31 and S32 carry out the operation of zero-voltage switching. The second module (or auxiliary circuit) comprises the following structure.
A third rectifying device having an anode coupled to the first terminal of the second switching device, and a cathode.
A fourth rectifying device having an anode and cathode coupled to the anode of the first rectifying device and the cathode of the third rectifying device respectively.
A fifth rectifying device having a cathode coupled to the cathode of the second rectifying device, and an anode.
A sixth rectifying device having a cathode coupled to the anode of the fifth rectifying, and anode.
An auxiliary capacitor having two terminals coupled to the cathode of the third rectifying device and anode of the fifth rectifying device respectively.
An auxiliary inductor having two terminal coupled to the cathode of the third rectifying device and the anode of the sixth rectifying device respectively.
An auxiliary switching device having a first terminal and a second terminal coupled to the anode of the sixth rectifying device and the second terminal of the second switching device respectively, and an enable terminal En4.
It is noted that the auxiliary switching device turns on before either the first switching device or the second switching device turn on.